Metallocene and non-metallocene single-site catalysts (hereinafter all referred to as single-site catalysts) provide olefin polymers with narrow is molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of a-olefin comonomers, low density, and controlled content and distribution of long-chain branching. Because of these unique properties, the polymers often outperform polyolefins prepared with Ziegler-Natta catalysts.
Polymerization of an olefin with a single-site catalyst is usually conducted in solution. Solution polymerizations are easy to control, and they can be carried out under a broad range of process conditions. However, solution polymerizations require polymer products to be soluble in process solvents. For this reason, solution polymerization is usually suitable for making very low density polyethylene (VLDPE), plastomers, or elastomers that are soluble in hydrocarbons. Solution polymerization is usually not suitable for making polymers with poor solubility in hydrocarbons, such as high-density polyethylene (HDPE) or polypropylene (PP).
HDPE, PP and other olefin polymers of higher density and crystallinity are usually made in a continuous slurry, fluidized-bed gas phase, or bulk polymerization. In these polymerization processes, the catalyst and the polymer products are neither soluble in the process solvent nor in the monomer at the reaction temperature employed. Smooth and continuous operation of the slurry or gas phase process requires the catalyst particles not to clog or clump, and not to foul the reactor wall, the agitator blades, or the distributor plates. While Ziegler-Natta catalysts are commonly used in slurry and gas phase processes, single-site catalysts are generally not used because they are soluble either in the process solvent or in the monomer.
The importance of non-solution processes for making polyolefins has sparked efforts to immobilize single-site catalysts while retaining their "single-site" nature. Because the perception exists that the supported single-site catalyst can be leached from the support in the reaction solvent employed, one approach to immobilizing the metallocene complex is to covalently bind it to the support (see, e.g., liskola et al., Macromolecules 30 (1997) 2853, or Lee et al., Macromol. Rapid Commun. 18 (1997) 427). Another method is to synthesize an amine-functional support by reacting partly hydroxylated silica with 3-aminopropyltrimethoxysilane, and then reacting the support with (.sub.5 Me.sub.5)TiCl.sub.3 to give a tethered metallocene catalyst (see Uozumi et al., Macromol. Rapid Commun. 18 (1997) 9). Amine-functionalized polystyrene has also been used to make supported imidovanadium catalysts useful for ethylene polymerization (Chen et al., J. Chem. Soc., Chem. Commun. (1998) 1673).
"Constrained geometry" or "open architecture" catalysts are known (see, e.g., U.S. Pat. No. 5,026,798 and EP 416,815). These unsupported catalysts comprise a metal complex of a cyclopentadienyl ring and a heteroatom bridged by a covalent group, for example, Me.sub.2 Si(C.sub.5 Me.sub.4)(N-tBu)TiCl.sub.2. These catalysts are highly active, and they enhance incorporation of long-chain .alpha.-olefin into ethylene polymers.
New single-site catalysts are needed. Especially needed are supported single-site catalysts that can be used in non-solution olefin polymerizations without sacrificing their "single-site" character. Preferably, the catalysts would remain anchored to the support throughout the polymerization and would avoid reactor fouling. Valuable catalysts would have high activity and would give polyolefins with narrow molecular weight distribution, good long-chain .alpha.-olefin incorporation, and a controlled degree of long-chain branching. Ideally, the catalysts would be easy and inexpensive to prepare with readily available reagents.